Chemistry

Name: _________________________

Electrolysis of Aqueous Solutions (Type II Cells) – [email protected]

Purpose – -

To electrolyze solutions of K2SO4, NaI and CuSO4 with graphite electrodes. To use observations to interpret the products of each electrolytic cell with anodic and cathodic half-reactions.

Materials – Apparatus – Two pencils, sharpened at both ends, one 9 V transistor battery, one battery cap with leads attached to “alligator clips”, one plastic petri dish, electricians or duct tape. Reagents – Approximately 10 mL each of 0.25 M K2SO4, 0.25 M NaI and 0.25 M CuSO4. Approximately 5 drops each of Universal Indicator and phenolphthalein. Procedure – Clean electrodes (pencil “lead”) before and after each use by wiping them with a paper towel or sharpening the pencils with a sharpener. Pour approximately 10 mL of each solution in turn along with the suggested indicator into a plastic petri dish. (K2SO4 with U-Indicator, NaI with phenolphthalein and CuSO4 with no indicator) Electrolyze for several minutes. The products show best against a white background with minimal movement of the dish. The remaining materials should be placed in the disposal container. Data and Observations – Complete a table such as the following for each electrolytic cell: Part I: Electrolysis of potassium sulfate solution Electrode Macroscopic Observations at Electrode Anode Cathode

Half Reaction at Electrode

Chemistry

Name: _________________________

Part II: Electrolysis of sodium iodide solution Electrode Macroscopic Observations at Electrode Anode Cathode

Half Reaction at Electrode

Part III: Electrolysis of copper(II) sulfate solution Electrode Macroscopic Observations at Electrode Anode Cathode

Half Reaction at Electrode

Questions – Explanations & Applications 1. For each of the three parts, how do the anode and cathode half-reactions explain what you observed? 2. The electrolytic cell in part II resembles the reactions that occur in a chloralkali plant where a NaCl solution is electrolyzed with inert electrodes. a) Predict the anode and cathode half-reactions that occur in a chloralkali plant. (Be sure to indicate which reaction occurs at the anode/cathode.) b) Suggest a reason why it would be unwise to simulate the chlor-alkali process in our laboratory. c) Examine the products at the electrodes. What common acid is manufactured as a byproduct at chlor-alkali plants? 3. What common electroplating process would be impossible if the “overpotential effect” did not exist? Explain why. 4. The electrorefining of copper produces highly pure copper metal at the cathode from an impure metal sample containing impurities such as lead, silver and zinc as well as copper. a) Consult a reference (be sure to cite your source) and draw a labeled diagram to illustrate an electrorefining cell. Be sure to identify the materials used as the anode, cathode and electrolytic solutions. b) Give at least two reasons why impurities don’t contaminate the copper collected at the cathode. Conclusion – Summarize what you have learned about electrolytic cells as a result of performing this investigation. Do not simply restate your observations.